<html xmlns="http://www.w3.org/1999/xhtml" 
      xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

<head>
<script>

function boom()
{
  document.getElementById("tree").firstChild.data = "xyz";
}

</script>
</head>

<body onload="boom();">

<xul:tree id="tree">a</xul:tree>

</body>
</html>
